Add Wagtail 7.4 to test matrix

[g-nie]

Adds Wagtail 7.4 to the tox test matrix (under Django 5.2 and 6.0), and pins django60 to Django>=6.0,<6.1.
Also drops py310/311 from the Django 6.0 matrix since those version combinations aren't supported.

Note: The pending test (3.9)–test (3.12) required Expected checks are probably stale branch protection entries, we might need to adjust the branch protection settings.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.0%. Comparing base (03cff96) to head (ca4b3c1).

Additional details and impacted files

@@          Coverage Diff          @@
##            main     #69   +/-   ##
=====================================
  Coverage   90.0%   90.0%           
=====================================
  Files          3       3           
  Lines         30      30           
  Branches       1       1           
=====================================
  Hits          27      27           
  Misses         2       2           
  Partials       1       1           

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